The present invention relates to a color picture tube used for a television set or a computer display, and more particularly relates to a shadow mask used for controlling the arrival position of an electron beam that is emitted from an electron gun.
As shown in FIG. 5, in general, a color picture tube comprises a face panel 2, a funnel 3, an electron gun 5, a deflection yoke 6 and a shadow mask 7. The face panel 2 is substantially rectangular and has a phosphor screen surface 2a formed inside thereof. The funnel 3 is connected to the rear part of the face panel 2. The electron gun 5 is provided inside of a neck portion 3a of the funnel 3. The deflection yoke 6 is attached to the periphery of the funnel 3. The shadow mask 7 is provided inside of the face panel 2 facing the phosphor screen surface 2a. 
The shadow mask 7 has a plurality of electron beam through apertures which are arranged regularly, and has the function of sorting three electron beams 4 emitted from the electron gun 5 based on color. press mask as shown in FIG. 6(a) that is formed by pressing to form a predetermined curved surface beforehand, a flat mask as shown in FIG. 6(b) that is formed by using a flat member without processing, a cylindrical mask as shown in FIG. 6(c) that is formed by curving a flat member along the longer axis or along the shorter axis, a shadow mask comprising a frame and a plurality of stripe-shaped materials, wherein the plurality of stripe-shaped materials are pulled with tension and are welded to fix to the frame (refer to Unexamined Japanese Patent Application No. Tokkai Hei 4-163830) and the like are well known as examples of a shadow mask.
Recently, a face panel constituting a color picture tube tends to be flattened because the flat face panel is not affected by outside light and the appearance of a color picture tube comprising the flat face panel is sophisticated. As a face panel tends to be flattened, its shadow mask also tends to be flattened. Therefore, when electron beams strike the shadow mask, the energy of the electron beams is converted into thermal energy, the shadow mask absorbs the thermal energy and the electron beam through apertures formed on the shadow mask are shifted due to the thermal expansion of the shadow mask caused by absorbing the thermal energy. As a result, the electron beams that pass through the electron beam through apertures can""t strike the phosphor substance accurately so as to cause irregularity of a color. That is, a so-called doming phenomenon occurs. Furthermore, as the shadow mask is flattened greatly, the degree of thermal expansion of the shadow mask becomes increased, and the problem of irregularity of a color caused by the doming phenomenon becomes more serious. In order to solve the above-mentioned problem, in a case of a flat mask and a cylindrical mask, tension that is stronger than the intensity of the thermal expansion caused by temperature increase of the shadow mask is applied in the directions shown by arrows as shown in FIGS. 6(b) and (c) so that positions of the electron beam through apertures formed on the shadow mask are not shifted from positions of phosphor dots formed on the phosphor screen surface which are supposed to be matched to each other.
However, in a case of a conventional tension mask such as a flat mask and a cylindrical mask, during the operation of a color picture tube, sufficient tension is applied not to cause the doming phenomenon of the shadow mask and prevent electron beams from arriving at a position that is shifted from a predetermined position due to the doming phenomenon of the shadow mask. Therefore, a heavy and strong support frame for holding a shadow mask is required and the weight of a color picture tube itself increases.
On the other hand, in a case of a color picture tube comprising a press mask as a shadow mask, a pressing mold and a pressing machine are required for producing the press mask, and therefore the cost for producing the color picture tube can""t be reduced.
It is the object of the present invention to overcome the problems of the prior art by providing (i) a color picture tube, wherein a light-weight support frame for holding a shadow mask can be used and the weight of the color picture tube can be reduced, and (ii) a color picture tube comprising a shadow mask that can be produced without using a pressing mold and a pressing machine.
In order to achieve the above object, a configuration of a color picture tube according to the present invention comprises a face panel having a phosphor screen surface formed inside, a funnel that is connected to the rear part of the face panel, an electron gun that is provided inside of a neck portion of the funnel and a shadow mask that has a plurality of electron beam through apertures for passing electron beams emitted from the electron gun. The shadow mask is positioned a predetermined distance from the phosphor screen surface. The tension applied to the shadow mask is such that the shadow mask has a tension that is 5-90% of the tension that would be applied to the shadow mask to prevent electron beams from arriving at a position that is shifted from a predetermined position of the phosphor screen surface due to the thermal expansion caused by absorbing thermal energy that is converted from the energy of electron beams when electron beams strike the shadow mask.
According to the above-mentioned configuration of the color picture tube of the present invention, the distance by which electron beams shift between a predetermined arrival position and an actual arrival position on the phosphor screen surface is reduced enough so as to matter for practical use. Consequently, the degree of irregularity of a color becomes irrelevant for practical use. As a result, a light-weight frame having the low strength can be used for holding the shadow mask with tension. Consequently, the weight of a color picture tube itself can be reduced and the design of the color picture tube can be simplified. Furthermore, a conventional large-scale machine for holding a shadow mask with tension is not required because the intensity of the tension that is applied to the shadow mask is small. Furthermore, unlike the conventional case producing a press mask, the shadow mask can be produced easily without using a pressing mold and a pressing machine.
In the above-mentioned configuration of the color picture tube according to the present invention, it is preferable that the tension applied to the shadow mask is such that the shadow mask has a tension that would be 40-60% of the tension that would be applied to the shadow mask to prevent electron beams from arriving at a position that is shifted from a predetermined position of the phosphor screen surface due to the thermal expansion caused by absorbing thermal energy that is converted from the energy of electron beams when electron beams strike the shadow mask. According to this preferable example, irregularity of a color can be further avoided.
In the above-mentioned configuration of the color picture tube according to the present invention, it is preferable that the tension is applied to the shadow mask so that the maximum distance by which electron beams shift between a predetermined arrival position and an actual arrival position on the phosphor screen surface due to the thermal expansion of the shadow mask caused by absorbing thermal energy that is converted from the energy of electron beams when electron beams strike the shadow mask is 100 xcexcm or less. According to this preferable example, during the broadcast of television and under the conditions of ordinary use of the display, the level of the irregularity of a color can be reduced so as not to matter for practical use.
In the above-mentioned configuration of the color picture tube according to the present invention, it is preferable that the shadow mask is made of invar material and the tension that is applied to the shadow mask is 1-2 kg/mm2.
In the above-mentioned configuration of the color picture tube according to the present invention, it is preferable that a frame for attaching the shadow mask is further provided, and the shadow mask is firmly held under tension by the frame.
In the above-mentioned configuration of the color picture tube according to the present invention, it is preferable that a frame for attaching the shadow mask is further provided, and the frame has means for generating tension to hold a shadow mask. In this case, it is preferable that the frame comprises the upper frame and the lower frame, the spring members are provided to separate the upper and lower frames at the both sides of the pair of frames.
In the above-mentioned configuration of the color picture tube according to the present invention, it is preferable that a shadow mask is flat.
In the above-mentioned configuration of the color picture tube according to the present invention, it is preferable that a shadow mask is curved along the longer axis or along the shorter axis.
In the above-mentioned configuration of the color picture tube according to the present invention, it is preferable that the inner surface and the outer surface of the face panel are substantially flat.
In the above-mentioned configuration of the color picture tube according to the present invention, it is preferable that the inner surface and the outer surface of a face panel are curved.
In the above-mentioned configuration of the color picture tube according to the present invention, it is preferable that the inner surface and the outer surface of the face panel are curved along the longer axis or along the shorter axis.
In the above-mentioned configuration of the color picture tube according to the present invention, it is preferable that the outer surface of the face panel is flat and the inner surface of the face panel is curved.
In the above-mentioned configuration of the color picture tube according to the present invention, it is preferable that the outer surface of the face panel is flat and the inner surface of the face panel is curved along the longer axis or along the shorter axis.